What a smart mom you are! I can tell you from personal experience that dairy allergies are extremely common, and they can begin at any point in life. Wheat can also cause allergic symptoms to be more dramatic.
We are the only species that consumes the milk of a different species on a regular basis. Cow's milk is not natural to our bodies.
I was diagnosed with a sensitivity/allergy to dairy last February, and as soon as I stopped consuming dairy, wheat, and sugar, all of my symptoms disappeared, and they haven't resurfaced (except on the occasions when I "cheated" a little with dairy or wheat). My previous symptoms had been chronic pain (joints, migraines, fibromyalgia); lack of energy; irritability; a general feeling of stuffiness and discomfort in my sinuses and chest; and sudden, uncontrollable weight gain. I lived that way for years.
When my doctor suggested this diet, I was afraid I would die without my cheese and crackers. But, she told me to try it for two weeks and see if I didn't improve. I felt dramatically better within three days, and I've never looked back. Now I can't even stand the slightest taste of dairy or wheat in anything I eat. The only grain I eat is oats, which are approved by my interactive M.D.
Why not try whitholding dairy for two weeks? If your daughter's symptoms improve, you'll know you've hit on something.
Be sure to read labels and avoid anything that contains whey or casein, both dairy derivatives. And remember that this is good for the whole family, not just one family member!
Good luck!